JavaScript is designed on a simple object-based paradigm. An object is a collection of properties, and a property is an association between a name and a value. A property's value can be a function, in which case the property is known as a method. In addition to objects that are predefined in the browser, you can define your own objects.

This chapter describes how to use objects, properties, functions, and methods, and how to create your own objects.

Objects overview

Objects in JavaScript, just as many other programming languages, can be compared to objects in real life. The concept of objects in JavaScript can be understood with real life, tangible objects.

In JavaScript, an object is a standalone entity, with properties and type. Compare it with a cup, for example. A cup is an object, with properties. A cup has a color, a design, weight, a material it is made of, etc. The same way, JavaScript objects can have properties, which define their characteristics.

Objects and properties

A JavaScript object has properties associated with it. A property of an object can be explained as a variable that is attached to the object. Object properties are basically the same as ordinary JavaScript variables, except for the attachment to objects. The properties of an object define the characteristics of the object. You access the properties of an object with a simple dot-notation:

objectName.propertyName

Like all JavaScript variables, both the object name (which could be a normal variable) and property name are case sensitive. You can define a property by assigning it a value. For example, let's create an object named myCar and give it properties named make, model, and year as follows:

Properties of JavaScript objects can also be accessed or set using a bracket notation. Objects are sometimes called associative arrays, since each property is associated with a string value that can be used to access it. So, for example, you could access the properties of the myCar object as follows:

An object property name can be any valid JavaScript string, or anything that can be converted to a string, including the empty string. However, any property name that is not a valid JavaScript identifier (for example, a property name that has a space or a hyphen, or that starts with a number) can only be accessed using the square bracket notation. This notation is also very useful when property names are to be dynamically determined (when the property name is not determined until runtime). Examples are as follows:

You can use the bracket notation with for...in to iterate over all the enumerable properties of an object. To illustrate how this works, the following function displays the properties of the object when you pass the object and the object's name as arguments to the function:

So, the function call showProps(myCar, "myCar") would return the following:

myCar.make = Ford
myCar.model = Mustang
myCar.year = 1969

Object everything

In JavaScript, almost everything is an object. All primitive types except null and undefined are treated as objects. They can be assigned properties (assigned properties of some types are not persistent), and they have all characteristics of objects.

Enumerating all properties of an object

Starting with ECMAScript 5, there are three native ways to list/traverse object properties:

for...in loops
This method traverses all enumerable properties of an object and its prototype chain

Object.keys(o)
This method returns an array with all the own (not in the prototype chain) enumerable properties' names ("keys") of an object o.

This can be useful to reveal "hidden" properties (properties in the prototype chain which are not accessible through the object, because another property has the same name earlier in the prototype chain). Listing accessible properties only can easily be done by removing duplicates in the array.

Creating new objects

JavaScript has a number of predefined objects. In addition, you can create your own objects. In JavaScript 1.2 and later, you can create an object using an object initializer. Alternatively, you can first create a constructor function and then instantiate an object using that function and the new operator.

Using object initializers

In addition to creating objects using a constructor function, you can create objects using an object initializer. Using object initializers is sometimes referred to as creating objects with literal notation. "Object initializer" is consistent with the terminology used by C++.

where obj is the name of the new object, each property_i is an identifier (either a name, a number, or a string literal), and each value_i is an expression whose value is assigned to the property_i. The obj and assignment is optional; if you do not need to refer to this object elsewhere, you do not need to assign it to a variable. (Note that you may need to wrap the object literal in parentheses if the object appears where a statement is expected, so as not to have the literal be confused with a block statement.)

Object initializers are expressions, and each object initializer results in a new object being created whenever the statement in which it appears is executed. Identical object initializers create distinct objects that will not compare to each other as equal. Objects are created as if a call to new Object() were made; that is, objects made from object literal expressions are instances of Object.

The following statement creates an object and assigns it to the variable x if and only if the expression cond is true:

if (cond) var x = {hi: "there"};

The following example creates myHonda with three properties. Note that the engine property is also an object with its own properties.

You can also use object initializers to create arrays. See array literals.

In JavaScript 1.1 and earlier, you cannot use object initializers. You can create objects only using their constructor functions or using a function supplied by some other object for that purpose. See Using a constructor function.

Using a constructor function

Alternatively, you can create an object with these two steps:

Define the object type by writing a constructor function. There is a strong convention, with good reason, to use a capital initial letter.

Create an instance of the object with new.

To define an object type, create a function for the object type that specifies its name, properties, and methods. For example, suppose you want to create an object type for cars. You want this type of object to be called car, and you want it to have properties for make, model, and year. To do this, you would write the following function:

Notice that instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects rand and ken as the arguments for the owners. Then if you want to find out the name of the owner of car2, you can access the following property:

car2.owner.name

Note that you can always add a property to a previously defined object. For example, the statement

car1.color = "black";

adds a property color to car1, and assigns it a value of "black." However, this does not affect any other objects. To add the new property to all objects of the same type, you have to add the property to the definition of the car object type.

Using the Object.create method

Objects can also be created using the Object.create method. This method can be very useful, because it allows you to choose the prototype object for the object you want to create, without having to define a constructor function. For more detailed information on the method and how to use it, see Object.create method.

Inheritance

All objects in JavaScript inherit from at least one other object. The object being inherited from is known as the prototype, and the inherited properties can be found in the prototype object of the constructor.

Indexing object properties

In JavaScript 1.0, you can refer to a property of an object either by its property name or by its ordinal index. In JavaScript 1.1 and later, however, if you initially define a property by its name, you must always refer to it by its name, and if you initially define a property by an index, you must always refer to it by its index.

This restriction applies when you create an object and its properties with a constructor function (as we did previously with the Car object type) and when you define individual properties explicitly (for example, myCar.color = "red"). If you initially define an object property with an index, such as myCar[5] = "25 mpg", you can subsequently refer to the property only as myCar[5].

The exception to this rule is objects reflected from HTML, such as the forms array. You can always refer to objects in these arrays by either their ordinal number (based on where they appear in the document) or their name (if defined). For example, if the second <FORM> tag in a document has a NAME attribute of "myForm", you can refer to the form as document.forms[1] or document.forms["myForm"] or document.myForm.

Defining properties for an object type

You can add a property to a previously defined object type by using the prototype property. This defines a property that is shared by all objects of the specified type, rather than by just one instance of the object. The following code adds a color property to all objects of type car, and then assigns a value to the color property of the object car1.

Defining methods

A method is a function associated with an object, or, simply put, a method is a property of an object that is a function. Methods are defined the way normal functions are defined, except that they have to be assigned as the property of an object. Examples are:

where objectName is an existing object, methodname is the name you are assigning to the method, and function_name is the name of the function.

You can then call the method in the context of the object as follows:

object.methodname(params);

You can define methods for an object type by including a method definition in the object constructor function. For example, you could define a function that would format and display the properties of the previously-defined car objects; for example,

Then you can call the displayCar method for each of the objects as follows:

car1.displayCar();
car2.displayCar();

This produces the output shown in the following figure.

Figure 7.1: Displaying method output.

Using this for object references

JavaScript has a special keyword, this, that you can use within a method to refer to the current object. For example, suppose you have a function called validate that validates an object's value property, given the object and the high and low values:

When combined with the form property, this can refer to the current object's parent form. In the following example, the form myForm contains a Text object and a button. When the user clicks the button, the value of the Text object is set to the form's name. The button's onclick event handler uses this.form to refer to the parent form, myForm.

Defining getters and setters

A getter is a method that gets the value of a specific property. A setter is a method that sets the value of a specific property. You can define getters and setters on any predefined core object or user-defined object that supports the addition of new properties. The syntax for defining getters and setters uses the object literal syntax.

Starting in JavaScript 1.8.1, setters are no longer called when setting properties in object and array initializers.

The following JS shell session illustrates how getters and setters could work for a user-defined object o. The JS shell is an application that allows developers to test JavaScript code in batch mode or interactively. In Firefox you can get a JS shell by pressing Ctrl+Shift+K.

o.c — a setter that sets the value of o.a to half of the value o.c is being set to

Please note that function names of getters and setters defined in an object literal using "[gs]et property()" (as opposed to __define[GS]etter__ ) are not the names of the getters themselves, even though the [gs]et propertyName(){ } syntax may mislead you to think otherwise. To name a function in a getter or setter using the "[gs]et property()" syntax, define an explicitly named function programmatically using Object.defineProperty (or the Object.prototype.__defineGetter__ legacy fallback).

This JavaScript shell session illustrates how getters and setters can extend the Date prototype to add a year property to all instances of the predefined Date class. It uses the Date class's existing getFullYear and setFullYear methods to support the year property's getter and setter.

Obsolete syntaxes

In the past, JavaScript supported several other syntaxes for defining getters and setters. None of these syntaxes were supported by other engines, and support has been removed in recent versions of JavaScript. See this dissection of the removed syntaxes for further details on what was removed and how to adapt to those removals.

Summary

added later to any object at any time using a getter or setter adding method.

When defining getters and setters using object initializers all you need to do is to prefix a getter method with get and a setter method with set. Of course, the getter method must not expect a parameter, while the setter method expects exactly one parameter (the new value to set). For instance:

Getters and setters can also be added to an object at any time after creation using the Object.defineProperties method. This method's first parameter is the object on which you want to define the getter or setter. The second parameter is an object whose property names are the getter or setter names, and whose property values are objects for defining the getter or setter functions. Here's an example that defines the same getter and setter used in the previous example:

Which of the two forms to choose depends on your programming style and task at hand. If you already go for the object initializer when defining a prototype you will probably most of the time choose the first form. This form is more compact and natural. However, if you need to add getters and setters later — because you did not write the prototype or particular object — then the second form is the only possible form. The second form probably best represents the dynamic nature of JavaScript — but it can make the code hard to read and understand.

Prior to Firefox 3.0, getter and setter are not supported for DOM Elements. Older versions of Firefox silently fail. If exceptions are needed for those, changing the prototype of HTMLElement (HTMLElement.prototype.__define[SG]etter__) and throwing an exception is a workaround.

With Firefox 3.0, defining getter or setter on an already-defined property will throw an exception. The property must be deleted beforehand, which is not the case for older versions of Firefox.